In today's wireless communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A wireless communications network comprises network nodes, i.e. base stations or radio base stations, providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, are served in the cells by the respective network node and are communicating with respective network node. The wireless devices transmit data over an air or radio interface to the network nodes in UL, transmissions and the network nodes transmit data over an air or radio interface to the wireless devices in downlink, DL, transmissions.
Long Term Evolution, LTE, is a project within the 3rd Generation Partnership Project, 3GPP, to evolve the WCDMA standard. LTE provides advantages such as increased capacity, higher data peak rates and significantly improved latency. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex, FDD, and Time Division Duplex, TDD, operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing, OFDM, is used in a DL transmission from a network node to a wireless device. Single Carrier—Frequency Domain Multiple Access, SC-FDMA, is used in an UL transmission from the wireless device to the network node. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread, DFTS-OFDM.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element, RE, corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix, cp, which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain. LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
DL and UL transmissions are dynamically scheduled. For example, in each DL subframe, the network node transmits control information about to or from which wireless device data is transmitted and upon which resource blocks the data is transmitted. The control information for a given wireless device is transmitted using one or multiple Physical Downlink Control Channels, PDCCH. Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe, where n is the Control Format Indicator, CFI. Typically the control region may comprise many PDCCHs carrying control information to multiple wireless devices simultaneously. Similarly, in each uplink subframe, the wireless device may transmit control information using one or multiple Physical Uplink Control Channels, PUCCHs.
Carrier Aggregation
In LTE Release 10 standard, a Component Carrier, CC, bandwidth of up to 20 MHz is supported. This is the maximal carrier bandwidth for the earlier LTE Release 8 standard. Hence, an LTE Release 10 operation that is wider than 20 MHz is possible. To a wireless device of LTE Release 10 standard, this may appear as a number of LTE carriers. However, it may also be advantageous to assure that an efficient use of a wide carrier is also performed for legacy wireless devices, i.e. where legacy wireless devices may be scheduled in all parts of the wideband LTE Release 10 carrier. One way to do is by means of Carrier Aggregation, CA, as shown in FIG. 3. In the LTE Release 10 standard, up to 5 aggregated carriers is supported. Each carrier is limited in the Radio Frequency, RF, specifications to have one out of six bandwidths, namely, 6, 15, 25, 50, 75 or 100 RBs. These correspond to 1.4, 3, 5, 10, 15 and 20 MHz, respectively.
The number of aggregated CCs, as well as, the bandwidth of the individual CC may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL are the same. An asymmetric configuration refers to the case where the number of CCs in DL and UL are different. Note that the number of CCs configured in the network node may be different from the number of CCs as seen by a wireless device. For example, a wireless device may support more DL CCs than UL CCs, even though the network node offers the same number of UL CCs and DL CCs. CCs may also be referred to as cells or serving cells.
Particularly, in an LTE network, the CCs aggregated by a wireless device may be denoted Primary Cell, PCell, and Secondary Cells, SCells. The term “serving cell” may comprise both a PCell and SCells. The PCell is wireless device specific and may be viewed as “more important”. That is because vital control signalling and other important signalling is typically handled via the PCell. The CC configured as the PCell is the primary CC, whereas all other CCs are secondary CCs.
During initial access a wireless device of LTE Release 10 standard acts similarly to a wireless device of LTE Release 8. For example, upon successful connection to the network a wireless device may, depending on its own capabilities and the networks capabilities, be configured with additional CCs in the UL and DL. This configuration may be based on Radio Resource Control, RRC, signalling. Because of heavy signalling and rather slow speed of the RRC signalling, a wireless device may be configured with multiple CCs, even though not all of the CCs are currently being used. If a wireless device is activated on multiple CCs, it follows that the wireless device has to monitor all DL CCs, e.g. for a Physical Downlink Control Channel, PDCCH, and Physical Downlink Shared Channel, PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc. This will result in higher power consumption by the wireless device.
SCell Activation and Deactivation
As mentioned, CA was introduced in LTE Release 10, and with that the concept of SCells, i.e. additional resources which could be activated or de-activated, i.e. configured or de-configured, when needed. Each SCell is configured with a SCellindex, which is an identifier or a so-called Cell Index that is unique among all serving cells configured for a particular wireless device. The PCell always have Cell Index 0 and SCell may have a integer cell index of 1 to 7.
In LTE Release 8/9/10 standard, the network node and the wireless device use so called Medium Access, MAC, Control Elements, CEs, to exchange information, such as, for example, buffer status reports, power headroom reports, activation commands, etc. One of the areas where MAC CEs are used is for activation and de-activation of SCells. For example, an Activation/De-activation MAC CE is defined in section 6.1.3.8 in Release 10 of the LTE standard specification 3GPP TS 36.321. Here, the Activation/De-activation MAC CE comprises a single octet containing seven C-fields and one R-field. Each C-field corresponds to a specific SCellIndex and indicates whether the specific SCell is activated or de-activated. The wireless device will ignore all C-fields associated with cell indices not being configured. The Activation/De-activation MAC CE always indicates the activation status of all configured SCells, meaning that if the network node wants to activate a SCell it has to include all configured SCells, setting them to activated or de-activated even if their status has not changed.
PUCCH Formats in LTE
In LTE Release 8 standard, PUCCH format 1/1a/1b and PUCCH format 2/2a/2b are supported for Scheduling Request, SR, Hybrid Automatic Repeat reQuest acknowledgement, HARQ-A/N, and periodic Channel State Information, CSI, reporting. The PUCCH resource is represented by a single scalar index, from which the phase rotation and the orthogonal cover sequence (only for PUCCH format 1/1a/1b) are derived. The use of a phase rotation of a cell-specific sequence together with orthogonal sequences provides orthogonally between different wireless devices in the same cell transmitting PUCCH on the same set of RBs.
In LTE Release 10 standard, PUCCH format 3 was introduced for CA for FDD and TDD, when there are multiple downlink transmissions, e.g. either on multiple carriers or multiple downlink subframes, but single uplink, e.g. either single carrier or single uplink subframe, for SR. HARQ-ACK, and/or CSI feedback. Similar to the other PUCCH formats, the PUCCH format 3 resource is also represented by a single scalar index from which the orthogonal sequence and the resource-block number may be derived. A length-5 orthogonal sequence is applied for PUCCH format 3 to support code multiplexing within one RB pair and a length-4 orthogonal sequence is applied for a shortened PUCCH. The PUCCH format 3 resource is determined according to higher layer configuration and a dynamic indication from the DL assignment. In detail, the Transmit Power Control, TPC, field in the Downlink Control Information, DCI, format of the corresponding PDCCH/EPDCCH is used to determine the PUCCH resource values from one of the four resource values configured by higher layers. For FDD, the TPC field corresponds to the PDCCH/EPDCCH for the scheduled secondary Scells. For TDD, the TPC field corresponds to the PDCCH/EPDCCH for the PCell with Downlink Assignment Index, DAI, value in the PDCCH/EPDCCH larger than ‘1’. A wireless device shall assume that the same PUCCH resource values are transmitted in each DCI format of the corresponding PDCCH/EPDCCH assignments.
In 3GPP, up to the Release 12 standard, the maximum number of downlink component carriers is 5. For HARQ-ACK feedback, PUCCH format 1b with channel selection and PUCCH format 3 have enough capability to feedback the HARQ-ACK for all configured carriers. However, in Release 13 standard, maximum 32 downlink carriers may be configured for one wireless device and hence at least one new PUCCH format will be introduced to carry more HARQ-ACK bits due to the aggregation of 32 DL CCs.